In machines, such as earth moving equipment having hydraulic implement systems, it is desirable that a dedicated hydraulic system be employed to charge a braking system. Typically, the dedicated hydraulic system directs pressurized fluid to mechanical brake assemblies which, in turn, are attached to ground engaging wheels to reduce the ground speed of the machine. Additionally, these machines also employ an auxiliary or secondary hydraulic system to drive a hydraulically operated fan motor, for example, to control the temperature of heat generating equipment such as an internal combustion engine.
It is known to combine various hydraulic implement control systems into a common hydraulic circuit such that a single source of pressurized fluid is provided to animate the various implement systems. For example, U.S. Pat. No. 6,314,729 B1, issued Nov. 13, 2001 to Crull et al. discloses a fan drive system including a pump hydraulically connected to a load sense circuit which provides fluid to first and second work circuits in addition to supplying fluid to a hydraulically driven fan unit. The fan drive system employs an electronic controller which, depending on the sensed temperature to be controlled, directs an electrical current to a proportional valve to modify the pressure drop across the fan motor.
However, the system disclosed by Crull et al. may lack suitable response performance since the signal circuitry of the proportional valve provides flow-modifying feedback to the load sensing pump and the proportional relief valve through the supply line. As a result, the fan motor may continue to run at an unwarranted level due to response performance. In fact, it has become imperative that the fan motor operates as sparingly as acceptable since the fan drive unit typically emits significant levels of noise which are undesirable to the operator. Moreover, the load sensing signals directed to the pump are similarly configured such that load communication between the work circuits, the fan drive system and the pump cause lethargic circuit response and as a result the system may be prone to inefficient operation. Additionally, the fan drive system is in continuous communication with the pump through a pressure reduction valve which leads to inefficient operation of the fan circuit. Such inefficient operation typically results in increased costs associated with ineffective operation and increased maintenance of the fan drive system in addition to the unwarranted noise generated by such a system.
Accordingly, it would be desirable to provide an efficient hydraulic fan drive system which may overcome one or more of the problems or disadvantages as set forth above.